Background: Despite the clinical importance of substance-induced psychosis (SIP), few studies have examined the course of this condition after its acute manifestation.

Objective: To investigate the rate of SIP conversion to a schizophrenia spectrum disorder and the length of follow-up needed to catch the majority of these patients whose diagnoses change. In addition to the conversion rate and pattern, we wanted to look for possible related factors.

Method: Using the nationwide Finnish Hospital Discharge Register, we followed all patients (N=18,478) since their first inpatient hospital admission with a diagnosis of SIP (codes 2921 and 2928 in DSM-III-R and codes F10–F19 in ICD-10 with a third digit of 4, 5, or 7) between January 1987 and December 2003 in Finland. Patients (mean age =43.7 years, standard deviation =13.5 years) were followed until first occurrence of schizophrenia spectrum disorder, death, or the end of December 2003, whichever took place first. Conversions of discharge diagnoses into schizophrenia spectrum disorders (codes 2951–2959 and 2971 in DSM-III-R and codes F20, F22, and F23 in ICD-10) were recorded at follow-up.

Results: Eight-year cumulative risk to receive a schizophrenia spectrum diagnosis was 46% (95% CI, 35%–57%) for persons with a diagnosis of cannabis-induced psychosis and 30% (95% CI, 14%–46%) for those with an amphetamine-induced psychosis. Although alcohol-induced psychosis was the most common type of SIP, 8-year cumulative risk for subsequent schizophrenia spectrum diagnosis was only 5.0% (95% CI, 4.6%–5.5%). No differences were detected with regard to gender, except for amphetamine-induced psychosis, which converted into a schizophrenia spectrum disorder significantly more often in men (P=.04). The majority of conversions to a schizophrenia spectrum diagnosis occurred during the first 3 years following the index treatment period, especially for cannabis-induced psychosis.